On-line measurement of the concentration of active halogens in aqueous and non-aqueous solutions is of great importance for many industrial and civil operations, such as halogenation reactions, bromine and chlorine production and disinfection of water or sewages by hypochlorites or hypobromites.
In principle, determination of active halogens may be accomplished by one of the following methods:
a. Titrimmetric analysis
Samples of the analyzed solution are reacted with a reducing reagent, generally sodium thiosulphate. The equivalent point is detected either by a change of color or by an abrupt change in the potential of a platinum electrode. The titration can be carried out either manually or by commercial automatic titrators. However, this is not an on-line method and even when performed automatically not more than 6-10 determinations per hour can be accomplished.
b. Potentiometry
The potential of a platinum electrode is measured in comparison to a reference electrode. This potential, according to the Nernst equation, depends on the concentrations of both the active halogen and the halide. Therefore, unless the halide concentration is constant, this method is essentially qualitative. Daily calibration is required in order to compensate for drifts in the potential of the reference electrode.
c. Photometry
This method is applicable when dealing with clear solutions that contain a single colored active halogen (I.sub.2, Br.sub.2) and provided that other constituents of the solution do not absorb in the same wavelength range as the active halogen. Even then, this method is restricted to a narrow range of concentrations, as the practical measuring range of absorbance is 0.2-1.5. The necessary instrumentation is delicate and expensive.
d. Voltammetry
Voltammetry includes a variety of electrochemical methods in which the current-potential dependence is measured. Under proper conditions, which include constant hydrodynamic conditions and sufficiently large overpotential, diffusion controlled current that is proportional to the concentration of the electroactive species and is independent of potential, may be attained. This principle is implemented in various electroanalytical methods such as polarography and RDE (Rotating Disc Electrode). Voltammetry, therefore, may provide a straightforward means for the measurement of active halogen concentrations.
Constant hydrodynamic conditions in a voltammetric cell can be sustained either by forced flow of the solution or by a constant movement of the electrode (rotation or vibration).
Voltammetric cells may be of the 2 or 3-electrode type. In the cells of the second type, current flows between two electrodes, called working and counter (auxiliary) electrode (hereinafter designated sometimes by the abbreviations WE and CE respectively), while a preset potential difference between the working electrode and a third electrode, called reference electrode (RE), is maintained. For this mode of operation a special power-supply, called potentiostat, is needed. Potentiostats are discussed e.g. in Allen J. Bard, Larry R Faulkner, "Electrochemical Methods-Fundamentals and Applications", John Wiley & Sons, 1980, p. 563.
The last three methods (b, c, d) can be applied in an "on-line" mode, however only in voltammetry the directly measured signal (current) is proportional to the concentration.
Industrial plants frequently use two-wire transmitters for monitoring various parameters such as temperature, conductance, etc., at points along the production line. The information from the transmitters is fed into a computer which may control the process. The checking points are some time located at long distances from the control station, with no access to the mains. The wires supply the energy required for the transmitter (16 to 40 Volts, 4 mA minimal current) and, at the same time feed back the information by changing the current flowing through the wires. Thus the transmitter is, virtually a current generator adjusted to the range of 4 to 20 mA for the minimal and maximal signals expected, respectively, regardless of the potential applied (from 18 to 40 volts, an error of 0.1% in the current reading is allowed). The transmitter is also unaffected by line noise excluding noise generated by the transducer proper.
Such transmitters are, in general, cheap electronic devices, manufactured by many companies. They require, however, a transducer and some electronic interfacing circuitry. Many companies offer various types of transmitters, which include the transducer and the current generator, all enclosed in one case. The most popular units available are the temperature, pH and conductivity transmitters. None of them, however, is adapted to potentiostatic systems.